The present disclosure relates to a power supply device that includes a DC-DC converter and a method of controlling such a power supply device. The present disclosure also relates to an image forming apparatus that includes the power supply device.
In an image forming apparatus such as a printer, a multifunctional peripheral, a copying machine or a facsimile machine, one or a plurality of printed circuit boards in which various circuits, elements and electronic components are incorporated are provided. For these circuits, elements and electronic components, the magnitudes (input voltage ranges) of drive voltages are individually and previously determined. A plurality of types of voltages may need to be input to one circuit, one element, one electronic component or one printed circuit board. Hence, within the image forming apparatus, a plurality of types of voltages need to be generated. In order to generate necessary voltages, a DC-DC converter is provided in the image forming apparatus. A DC-DC converter below is known.
Specifically, the DC-DC converter is known which includes a switching element, an inductor, a diode, an output capacity and a control circuit and in which when an output voltage is lowered beyond a predetermined voltage value, an inductor value is decreased with an inductor switching means. In this configuration, the DC-DC converter does not degrade performance in a stably operated state, and decreases the time during which an inductor current is increased only when a load current is increased and the output voltage is lowered, and thus the undershoot of the output voltage is reduced.
In the DC-DC converter, an overcurrent protection circuit may be provided. The overcurrent protection circuit detects that a current flowing through the DC-DC converter exceeds a predetermined upper limit value by a factor such as an increase in the load current. When the current exceeds the upper limit value, the overcurrent protection circuit stops the DC-DC converter.
The elements and components of the DC-DC converter which do not break down even when the current of the upper limit value flows through them are used (which correspond to the upper limit value), and thus a failure and a damage in the DC-DC converter caused by an overcurrent are unlikely to occur. However, when the elements and components of the DC-DC converter are selected so as to correspond to the upper limit value of the current, the elements and components may be larger than necessary and may exceed the specifications.
Specifically, when a coil is selected so as to correspond to the upper limit value of a current which flows through the DC-DC converter, in terms of the magnitude of a current supplied to a load (a current value at the time of an actual operation), the coil may be larger than necessary and may be expensive. A coil in which a current value (temperature increase allowable current value) determined such that when a current beyond this current is passed, a damage caused by heating occurs is equal to or more than the upper limit value and a coil in which a current value (direct-current superimposition allowable current value) where the lowering of an inductance is increased is equal to or more than the upper limit value may be larger than necessary in terms of the output current value of the DC-DC converter at the time of the actual operation. When a coil is selected so as to correspond to the upper limit value of a current which flows through the DC-DC converter, this increases the size of the DC-DC converter itself, the size of an apparatus including the DC-DC converter and the manufacturing cost. In the following description, any one of the “temperature increase allowable current value” and the “direct-current superimposition allowable current value” or one of them which is lower is referred to as an “allowable current value”.
A coil whose allowable current value is lower than the upper limit value of the current which flows through the DC-DC converter and which corresponds to the magnitude of an actual current supplied by the DC-DC converter to a load (the current value at the time of the actual operation) may be adopted so that the size and cost of the coil are prevented from being increased.
Here, when a state where the current which flows through the coil of the DC-DC converter is high (the load current is high) is continued for a long period of time, the temperature of the coil is increased. In this way, the inductance is lowered, and thus a current easily flows. When the temperature of the coil is increased as a current exceeding the allowable current value is passed, the DC-DC converter is preferably stopped. However, in a case where a coil (coil whose allowable current value is lower than the upper limit value of the current flowing through the DC-DC converter) which corresponds to the current value at the time of the actual operation is adopted, when an output current value is equal to or less than the upper limit value, the function of stopping the DC-DC converter with the overcurrent protection circuit does not work. Hence, disadvantageously, when the coil which corresponds to the current value at the time of the actual operation is adopted, even in a state where the temperature of the coil is significantly increased and where the operation of the DC-DC converter needs to be stopped, it is likely that the overcurrent protection circuit does not stop the DC-DC converter.
The known technology described above is not related to a problem in which the coil that corresponds to the current value at the time of the actual operation is adopted as the coil of the DC-DC converter. Hence, it is impossible to solve the above problem.